1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus using a solid-state image pickup device and, more particularly, to a solid-state image pickup apparatus which can control the exposure time.
2. Related Background Art
Hitherto, as shown in Japanese Laid-Open Patent Gazette No. 138371/1981, there has been considered a solid-state image sensor such as a CCD or the like in which in order to prevent blooming, the surplus carriers are extinguished by use of surface recombination in place of providing an overflow drain in the photo sensing surface.
Such an image sensor has advantages such that since the aperture ratio in the photo sensing surface is not sacrificed, the sensitivity is high and the integration degree can be improved, so that the horizontal resolution can be raised and the like.
FIGS. 1 to 3 are diagrams for explaining a method of preventing blooming by the foregoing surface recombination. FIG. 1 is a front view of a general frame transfer type CCD. In the diagram, reference numeral 1 denotes a photo sensing section consisting of a plurality of vertical shift registers having photo sensitive properties.
A storage section 2 consists of a plurality of vertical shift registers which are shielded against the light.
Numeral 3 denotes a horizontal shift register. By simultaneously shifting the data in each vertical shift register of the storage section 2 by one bit, the data is stored into the horizontal shift register. Next, by allowing the register 3 to perform the horizontal shift operation, the video signal can be obtained from an output amplifier 4.
In general, the data formed in each vertical shift register of the photo sensing section 1 is vertically transferred into the storage section 2 within the vertical blanking period in the standard television system. This data is sequentially read out one line by one from the horizontal shift register 3 within the next vertical scan period.
It is assumed that each of the photo sensing section 1, storage section 2, and horizontal shift register 3 is double-phase driven and their respective shift electrodes are P.sub.1, P.sub.2, P.sub.3, P.sub.4, P.sub.5, and P.sub.6, and their shift clock signals are (.phi..sub.p1, .phi..sub.p2), (.phi..sub.p3, .phi..sub.p4), and (.phi..sub.p5, .phi..sub.p6).
FIG. 2 is a diagram showing a potential profile under the shift electrodes P.sub.1 to P.sub.6. As shown in this diagram, each electrode is provided for a P-type silicon substrate 6 through an insulating layer 5. The portion having a low potential and the portion having a high potential with respect to the electrons are formed under each electrode by way of ion implantation or the like. When the low-level voltage XV.sub.1 is applied to the electrodes P.sub.2, P.sub.4, and P.sub.6 and the high-level voltage V.sub.2 is applied to the electrodes P.sub.1, P.sub.3, and P.sub.5, the potentials as shown by solid lines in the diagram are formed. On the contrary, when the low-level voltage V.sub.1 is applied to the electrodes P.sub.1, P.sub.3, and P.sub.5 and the high-level voltage V.sub.2 is applied to the electrodes P.sub.2, P.sub.4, and P.sub.6, the potentials as shown by broken lines in the diagram are formed.
Therefore, by applying the alternating voltages to the electrodes P.sub.1, P.sub.3, and P.sub.5 and to the electrodes P.sub.2, P.sub.4, and P.sub.6 in opposite phases, the carriers are sequentially transferred in one direction (to the right in the diagram).
In the diagram, an alternate long and short dash line denotes the potential in the case where a large positive voltage V.sub.3 is applied to the electrodes. Since the well of this potential is in the inverted state, the surplus carriers above a predetermined amount are recombined with the majority carriers and are extinguished.
FIGS. 3 is a diagram showing such an electrode voltage and a shape of internal potential with respect to the direction of thickness of the semiconductor substrate 6. As will be obvious from this diagram, the potential well is shallow to the electrode voltage V.sub.3 and the second state in which the surplus carriers are recombined with the majority carriers at the interface with the insulating layer is obtained.
On the other hand, at the electrode voltage -V.sub.1, the accumulation state is obtained as the first state. The majority carriers are likely to be collected around the interface. (For example, the majority carriers are supplied from the channel stopper region (not shown)).
Therefore, for example, by alternately applying the voltages -V.sub.1 and V.sub.3 to the electrode P.sub.1 in the state in which the barrier was formed by applying the voltage -V.sub.1 to the electrode P.sub.2, the minority carriers which are accumulated under the electrode P.sub.1 are suppressed to an amount below a predetermined amount.
However, the image sensor using such a charge recombination has an inconvenience such that the clock signal for recombination is mixed into the output signal and causes noise.
Therefore, in order to remove such clock signal moise, the following method has already been proposed by the same applicant as this application. This method will be explained in conjunction with FIG. 4. The frame transfer type CCD of the single-phase driving system will be described.
FIG. 4 is a diagram showing an electrode structure of the cross section in the boundary region of the photo sensing section 1 and storage section 2 and an outline of the potentials.
In the diagram P.sub.PI, denotes a shift electrode to apply the shift clock signal .phi..sub.PI of the photo sensing section; P.sub.AB denotes a recombination control electrode as recombining means to apply the recombination clock signal .phi..sub.AB ; P.sub.PS is a shift electrode to apply the shift clock signal .phi..sub.PS of the storage section; and 6E is an n.sup.+ region constituting the overflow drain. .phi..sub.AB indicated the clock signal to recombine the surplus charges with the holes at the center of the surface recombination.
The potential state indicated by the solid lines in the diagram is obtained when the low-level voltage was applied as .phi..sub.PI and .phi..sub.PS and the high-level voltage was applied as .phi..sub.AB. The broken line indicates the potential state which is obtained when the high-level voltage was applied as .phi..sub.PI and .phi..sub.PS and the low-level voltage was applied as .phi..sub.AB.
The stairway of the potentials as shown in the diagram is formed in the substrate 6 by way of ion implantation. Although not shown, for example, a P-type inverted phase to constitute the virtual electrode is formed under the insulating layer which is not covered by the electrodes P.sub.PI, P.sub.PS, and P.sub.AB, namely, in the boundary portion between the insulating layer and the semiconductor substrate.
Therefore, the potential in the semiconductor region which is not covered by the electrode is not changed depending on the bias to each electrode.
FIG. 5 is a diagram showing an example of an electrode pattern in the region shown in FIG. 4. In the diagram, CS denotes a channel stop to prevent the movement of the charges in the horizontal direction.
With the constitution shown in FIGS. 4 and 5, the width of the electrode P.sub.AB to recombine the charges can be set to be sufficiently smaller than the width of the shift electrode P.sub.PI, so that the efficiency of eliminating the surplus charges can be raised.
On the other hand, in the CCD image sensor of the single-phase driving system, the recombining operation of the charges can be performed independently of the transferring operation.
Moreover, the foregoing recombination controlling structure of the image sensor can be realized by the step of forming the polysilicon gate for electrodes which can be manufactured by the same process as that of the channel stop and by the ion implantation step of forming a stairway of the interval potentials.
With such a constitution, even if an object having remarkable luminance level exists in the pickup image, the bad influence such as a blooming or the like can be prevented.
However, by supplying an anti-blooming pulse to the image sensor, the electric power consumption of the sensor itself increases, so that additional heat is generated.
Therefore, in an image sensor where sufficient heat radiation measures are not taken, the dark current component increases. In particular, there occurs an inconvenience such that the S/N ratio deteriorates in the low-luminance portion.
To prevent such an inconvenience, a method whereby the frequency of the recombination pulse is set to a low frequency is considered. In this case, however, there occurs a drawback such that when the exposure time is short, the blooming is imperfectly eliminated.